1. Field of the Invention
This invention relates to a method for separating a mixture of finely divided minerals into constituents thereof. In particular, the invention relates to a procedure for effecting such separation by a selective flocculation technique wherein a composition comprising at least two components is employed as selective flocculation additive. Such components are added to a previously processed and conditioned aqueous slurry containing the mixture of minerals to produce a high yield product which is low in impurities. This invention is especially adapted to the separation of colored impurities, such as titania (anatase and rutile) and micas, from kaolin clay.
2. Description of Related Art
When particles of mineral ore or powder mixtures are sufficiently large, for example, larger than 325 (U.S.) mesh, the components of the mixture can be separated by simple physical means such as air or magnetic separation. When particles are finer, more sophisticated technology may be needed to bring about efficient separations. It is conventional to make the separation of finely divided mineral, e.g. particles finer than 325 mesh, by forming the mixture into an aqueous suspension or slurry and providing physical and/or chemical treatments that will bring about a desired separation.
One widely used treatment method is froth flotation. In the case of froth flotation of phosphate or oxidized minerals from siliceous gangue, it is conventional to use a fatty acid collector and a salt promoter. The fatty acid collector coats the mineral particles, which are separated from the siliceous gangue in the form of a froth, and a clay dispersant may be used to keep the clay particles dispersed during the froth flotation process. In the case of kaolin clays which contain significant amount of sub-micron contaminants, conventional froth flotation techniques may not produce the desired removal of colored bodies.
Selective flocculation is a procedure that is widely used commercially to separate finely divided minerals and powders. In the case of clay, some processes utilize anionic polymers to selectively flocculate the clay, leaving the impurities, such as titanium, in the form of titania, dispersed and amenable to subsequent separation from the clay. Commercial variants of selective flocculation employ weakly anionic polymers such as hydrolyzed polyacrylamide to selectively flocculate impurities in the clay, leaving the purified clay dispersed. See, for example, U.S. Pat. No. 3,837,482 to Sheridan; U.S. Pat. Nos. 3,701,417 and 3,862,027, both to Mercade; U.S. Pat. No. 3,371,988 to Maynard, et al.; and U.S. Pat. No. 4,604,369 (Shi).
A variant of the selective flocculation process is where the impurities in the clay are flocculated and the clay remains dispersed in the slurry. Examples of this process are disclosed in U.S. Pat. No. 3,857,781 assigned to Huber; U.S. Pat. No. 4,604,369 assigned to Thiele; and U.S. Pat. Nos. 5,535,890 and 5,603,411 assigned to Engelhard. These processes may employ a water soluble high molecular weight organic anionic polymer having a molecular weight in excess of 1 million, such as a co-polymer of acrylamide, e.g. an acrylamide and acrylate copolymer having a molecular weight in excess of 5 million.
U.S. Pat. No. 4,604,369 further teaches the use of ammonium chloride as an agent for conditioning the slurry to be treated and use of blunging, diluting, and flocculating steps carried out in a substantially continuous manner. U.S. Pat. No. 5,535,890 further teaches the use of a fatty acid and calcium chloride as the conditioning agents.
U.S. Pat. No. 5,603,411 uses a mixture of dispersants including soda ash to disperse a slimed ore pulp prior to conditioning with a fatty acid, salt, and then addition of a polymeric flocculating agent. U.S. Pat. No. 5,584,394 uses a selective flocculation process in which a dispersed aqueous mineral mixture is preconditioned by use of oleic acid and a polyvalent cation which is said to coat the titania, the coated titania is flocculated with high molecular weight anionic copolymer and sodium polyacrylate may be used to disperse kaolin pulp prior to conditioning or after flocculation.
To achieve selective adsorption of a flocculating agent on a particular component of a mixture, a number of methods have been suggested in the literature [Yu and Attia; in "Flocculation in Biotechnology and Separation Systems," (Y. A. Attia, ed.) p. 601, Elsevier, Amsterdam 1987; Behl, S. and Moudgil, B. M., Minerals and Metallurgical Processing, 5, 92,1992 and, Behl, S. and Moudgil, B. M., Journal of Colloidal Interface Science, 160, 1993]. One of the methods involves selectively blocking the active sites on the inert or non-flocculating component for adsorption of the polymeric flocculating agent. This may be achieved by adsorption of a lower molecular weight fraction of the flocculating agent, which can act as a dispersant and/or site blocking agent prior to exposing the particle surfaces to the flocculating agent.
The selective flocculation processes employing high molecular weight anionic polymers to bring about selective flocculation of the impurity particles generally suffer from the problem of low product yields due to the high molecular weight polymers attaching onto and settling with the oppositely charged cationic edges of the kaolin clay particles. Other disadvantages of the processes of this kind used in the prior art involve producing a product with a relatively high level of residual TiO.sub.2 such as greater than 0.60% and as high as 1% or more. Additional disadvantages are the formation of small flocs that require a long settling time (with a large volume selective flocculation vessel or `thickener` to maintain production) and the imparting of minimum shear on the slurry to allow settling. In addition, the presence of fatty acids because of their hydrophobicity may cause performance problems if present in the final product.
The purpose of the present invention is to provide an improved method for the treatment of an aqueous slurry of kaolin clay to separate impurities therefrom by selective flocculation in which the impurity particles are selectively flocculated and the kaolinite particles remain deflocculated, the method being such that the aforementioned disadvantages of the prior art are reduced or eliminated.